Process for the manufacture of chrome powder

ABSTRACT

A method for the industrial manufacture of chromium powder from chromium oxide powder by reducing the latter with magnesium vapor, including the steps of preparing chromium oxide powder masses of between one and about 10 kilograms, imbedding in each of said masses a magnesium ingot, thermally insulating the masses from each other, and heating the masses to react the magnesium vapor produced by the magnesium ingots with the chromium oxide powder.

United States Patent Gohin et al.

[54] PROCESS FOR THE MANUFACTURE [56] References Cited 0F CHROME POWDER UNITED STATES PATENTS 1 Inventors: Gilles Gohin; Andre both of 2,848,324 8/1958 Krapf ..75/27 Pomolsey France 2,997,165 8/1961 Leuthy et al.. ..75/27 [73] Assignee: Office National DEtudes Et De Recherches 3,232,749 2/1966 Yntema ..75/27 Aerospatiales, Chatillon-sous-Bagneux, France Primary Examiner-L, Dewayne Rutledge Assistant Examiner-W. W. Stallard [22] Flled' 1970 Att0rneyChaTles E. Temko [21] Appl. No.: 8,347

[57] ABSTRACT Related US. Application Data I A method for the industrial manufacture of chromium powder [62] Dmslo of 715,114 261 1963 from chromium oxide powder by reducing the latter with mag- 3,536,3()9. nesium vapor, including the steps of preparing chromium oxide powder masses of between one and about 10 kilograms, 2% F' 'i 36 5 33 imbedding in each of said masses a magnesium ingot, ther- 1 'i mally insulating the masses from each other, and heating the [58] Field of Search ..75/27, 0.5 B, 62

masses to react the magnesium vapor produced by the magnesium ingots with the chromium oxide powder.

4 Claims, 4 Drawing Figures o o o l0 0 o\ o o 6 l/ 23 o 9 /18 a I 22 a s 2 p o 20 24 O Patented A ril 25, 1972 oouowqqp oooanoonlplbo aooooo w W W W i PROCESS FOR'TI-IE MANUFACTURE OF CHROME POWDER This application is a division of our co-pending application.

Ser. No. 715,114 filed Feb. 26, 1968 under the same title now US. Pat. No. 3,536,309, dated Oct. 27, 1970.

The invention relates to a process for the manufacture of chromium powder.

It has already been proposed to manufacture chromium powder by subjecting chromium oxide powder to the action of magnesium in vapor form. In this way, starting from fine chromium oxide powder, sometimes called chromium green, chromium powder of a previously unobtainable degree of fineness and of great purity is obtained.

The object of this invention is to extend the process on an industrial scale, i.e. to make it possible to treat me single. operation masses of chromium oxide of sufficient bulkness for the cost to permit numerous and wide applications.

A particular object of the invention is to provide a process and an'apparatus for obtaining in a single operation chromium powder in several dozen kilograms or even more in spite of the highly exothermic character of the reaction.

Another object of the invention is to provide a method and an apparatus whereby an exothermic reaction can beccontrolled satisfactorily without prolonging excessively the reaction time.

A" general object of the invention is to provide a method and an apparatus for obtaining chromium powder of a much greater degree of fineness than that of ordinary industrial powder and at a competitive cost.

The method involves using magnesium in the most available form, i.e. in the form of ingots, and disposing the powder to be treated in the same receptacle as the ingot, or ingots, so that the working of the method is particularly simple.

The invention comprises steps that enable violence of the reaction to be effectively counteracted. This violence is liable to occur because of the exothermic character of the reaction. By these steps the temperature of the whole is limited to the desired value so that the vaporization of the magnesium is not accelerated beyond what is necessary.

According to the invention, for the industrial treatment of chromium oxide powder with magnesium in vapor form in order toobtain chromium powder, an industrial quantity of chromium oxide powder is divided into masses between 1 andabout kilograms, magnesium in ingot formis embedded in each of these masses, which are thermally insulated from each other, and the industrial quantity is heated in the same enclosure and in a single treatment phase.

Thus each portion of the mass is brought into the optimum reaction conditions without being subjected excessively to the influence of the heat liberated by the other portions simultaneously undergoing the reaction. According to the invention then, the process resultingin the production of chromium oxide powder is influenced by dividing the treated mass.

The invention makes it possible to take advantage simultaneously of the bulk of the mass treated in one operation, with the resulting economic advantages, and also the strictness of the control to which each of the portions can be subjected so that the reaction may proceed under optimumconditions, with, as a result, a complete or practically complete conversion of the'chromium oxide into chromium of exceptionally fine particle size.

In particular, the invention avoids the formation, in spite of the bulk of the total mass treated, of. conglomerates of various compositions resulting from excessive local raising of the temperature. These conglomerates otherwise appear inevitably when chromium oxide in a large mass is heat-treated with magnesium vapor.

The invention therefore provide ameans for'extendingthe industrial use ofmagnesium' as a reducingagent for oxides, such as Cr O whoseformation heat is not as great as'that of the oxides usually reduced'by magnesium, while keepingthe reduced metalin the state of a very fine powder.

In one embodiment of the invention, themass ofchromium oxide to be treated is distributed in crucibles or pots piled one on top of each other, means being provided for the provision of thermal insulation between the successive crucibles.

To provide this thermalinsulation, the invention contemplates the application of an insulating layer as a heat barrier between the portions contained in two successive crucibles. For this purpose it proposes, for example, divided magnesia or alumina in the form of a powder or of a sufficiently porous agglomerate.

The crucibles are in an inert atmosphere, for example. of argon, leak-proofness beingv ensured by a cooled seal remote from the treatment crucibles.

In the following description given by way of example reference is made to the accompanying drawings in which:

FIG. 1,is a diagrammatic vertical section through an apparatus according to the invention in one embodiment;

FIG. 2 is a diagrammatic section through a crucible according to the invention in one embodiment;

FIG. 3 is a view similar to FIG. 2.but for another embodiment; and

FIG. 4 is a diagrammatic horizontal section through another embodiment of the. apparatus according to the invention.

With reference to FIG. 1, the furnace 10, which is raisable and lowerable, has a wall 11 made of refractory material on whose inner face there is a heating resistor 12, which is ad vantageously divided into sections; the current supply of each section may be controlled independently by means indicated diagrammatically at C. In FIG. 1, the furnace rests on the bottom 13, thus bounding a cylindrical space 14. The treatment enclosure 15 is bounded by a bell-shaped member 16, made for example of refractory alloy and of a generally cylindrical shape, leak-proofness at the bottom edge of the bell-shaped member being provided by a known seal 17 made of rubber or the like and cooled by water circulation.

Crucibles or boxes 18,, 18 18 etc., for example about 10 in number, are piled one on top of the other, the crucible 18, resting by its bottom 19 on the top edge 20 of the crucible 18,, etc. The bottom crucible 18 rests on a base 21, keeping it away from the bottom of the furnace and the seal 17. Each of the crucibles 18 is made of graphite and contains in its interior an iron or mild-steel dish 22.

Heat insulation is provided betweentwo adjacent crucibles. In the embodiment described: heat insulation 242isprovided between the bottom 19 of each crucible, with the exception of the bottom crucible 18,, and the bottom 23 of the dish it contains. This insulation may consist of a layer of powdered alumina or magnesia. As a modification, a disc maybe provided that is formed by an agglomerate of a material that is sufficiently insensitive to magnesium vapors, such as magnesia or alumina, and has sufficient porosity to ensure good heat insulation. A porous-carbon disc may also be used.

A magnesium ingot 25 embedded in chromium oxide powder to be treated 26 is .placed in each of the receptacles 22. The chromiumoxide powder is commercial powder. Its fineness depends on the fineness of the chromium powder it is desired to obtain. The quantity of chromium oxide powder contained in each crucible may be between 1 and about 10 kilograms, and the magnesium mass is in accordance with the quantity of chromium oxide powder.

Argon, which forms the protective atmosphere, arrives at the bottom of the furnace through the passage 27'.

The current supply of each section of the heatingresistor 12. is controlled independently from a thermostatic element that is sensitive to the temperature of the reaction mass of the crucible or crucibles opposite the resistor section andltherefore mainly heated by the latter. This being so, provision may be made, for the different crucibles,,forthe observance of a predetermined law, for example in stages, of the variation of the temperature as.a function of time .towhich corresponds the progress desired for the reaction which starts at'about 800 C.

A programmer provides in known manner for the control of the electricity supply of the various resistor sectionstaking- During the heating, the graphite crucibles maintain their shape without creep in spite of the high temperatures they endure; they may be used for a considerable number of treatments.

When submicronic chromium oxide powder is used as starting material, chromium powder is obtained whose particles have an average diameter approximately between a fraction of a micron and several microns.

In the embodiment shown in FIG. 2, an iron or mild-steel dish 40, the bottom of which 41 is convex, for example spherical, contains the reaction mass and rests on a layer 24 of powdered refractory material.

In the embodiment in FIG. 3, the dish 22, which has a sub- 'stantially plane bottom, rests on a slab 42 made of sintered magnesia and held by an inner rim 43 of a graphite dish 44, the bottom of which thus has a large circular aperture 45.

In a variant of the apparatus according to the invention, the pots or crucibles may be placed side by side but separated thermally from each other. In this case heating is advantageously effected by the roof, the bottom or the roof and the bottom at the same time.

FIG. 4 relates to an apparatus according to the invention having four columns of crucibles 31,41 each of which is similar to the one described above with reference to FIG. 1. A central post 32 is provided; it may have partitions distributed regularly in the form of a cross with inwardly curved arms. The post and partitions are made of one or several materials adapted to effect good heat insulation. The crucible columns are thus thermally separated from each other.

The invention provides for the treatment with magnesium vapors of chromium oxide powders whose various particles are further away from each other than those of an ordinary chromium oxide powder. This promotes the action of the magnesium vapor and makes it more regular.

To obtain such a blown or thinned" powder, it is proposed to subject ordinary chromium oxide powder to turbining in a turbine grinder without a screen. This aeration operation reduces the apparent density from 20 to 30 percent. The resulting emulsion of chromium oxide powder in air has enough stability for it to be used with advantage several hours after its formation.

What is claimed is:

l. A method for the industrial manufacture of chromium powder from chromium oxide powder by reducing the latter with magnesium vapor, comprising the steps of preparing chromium oxide powder masses between one and about 10 kilograms, embedding in each said mass a magnesium ingot, thermally insulating from one another said chromium oxide powder masses containing magnesium ingots, and heating said masses to react the magnesium vapor produced by said magnesium ingots with said chromium oxide powder.

2. A method as claimed in claim 1, further comprising the steps of controlling the temperature of said masses during the reduction reaction step.

3. A method as claimed in claim 2, wherein said temperature control is effected individually with respect to each mass.

4. A method as claimed in claim 1, wherein said chromium oxide powder is blown prior to being separated into 1 to about 10 kilograms masses. 

2. A method as claimed in claim 1, further comprising the steps of controlling the temperature of said masses during the reduction reaction step.
 3. A method as claimed in claim 2, wherein said temperature control is effected individually with respect to each mass.
 4. A method as claimed in claim 1, wherein said chromium oxide powder is blown prior to being separated into 1 to about 10 kilograms masses. 